1. Field of the Invention
This invention relates to a method for controlling continuously variable drive ratio transmissions and a system therefor. More particularly, this invention relates to an improvement in controlling the continuously variable drive ratio transmission which is being employed in the driving system of vehicles driven by means of output power of an internal combustion engine.
2. Description of the Prior Art
According to the turbo-torque converter generally known as the continuously variable drive ratio transmission, the drive ratio is determined at each of its operating conditions. However, the continuously variable drive ratio transmission with which this invention is concerned is adapted to the conventional mechanical or hydraulic transmission of the continuously variable drive ratio which may be freely controlled outside of the transmission. The control of such transmission in accordance with the invention only by a single pedal will realize the use of the most effective zone of the internal combustion engine when the engine is utilized for driving the vehicle and the maximum dynamic braking generally referred to as "engine braking" when necessary.
It is a conventional practice to utilize the hydraulic transmission for effectively applying the dynamic brake. According to the conventional practice, it is necessary to provide a pedal exclusively for use in dynamic braking. Alternatively, the dynamic brake is applied by the depression of one foot pedal at the idle stage and then the usual mechanical brake is applied by the depression thereof at the further stage.
One of the features of this invention resides, however, in that the dynamic brake may be applied in proportion to the return of the acceleration pedal for driving the vehicle, after being depressed.
Before the explanation of the invention in detail, the relevant principles of operation of the internal combustion engine will be described. It is well known that the operational point of the internal combustion engine is determined when two of the following three conditions are determined: the output torque of the engine, the rotational speed thereof, and the depression of the acceleration pedal. More specifically, with reference to FIG. 1 wherein the characteristic of gasoline engine is shown, the vertical axis therein reflects the output horse power (PS) of the gasoline engine, while the horizontal axis reflects the rotational speed (rpm). In FIG. 1, the straight line T represents the characteristic of the constant output torque of the gasoline engine, the curve .theta. represents the throttle degree proportional to the depression of acceleration pedal, and the curve C represents the line reasonably interconnecting a point desirable for attaining the minimum fuel consumption at each throttle degree. Another curved line similar to the line l may be drawn in accordance with the various purposes other than the fuel consumption purpose, for instance, for the purpose of attaining the minimum exhaust of NOx or Co. It is, therefore, to be noted that the drive condition of each internal combustion engine may be selected for achieving each purpose so that the engine can be operated in the most desirable way.
In accordance with the above, when the throttle degree is given, the engine operational point P will be determined by setting the rotational speed thereof to the point indicated as "no". In addition, the throttle degree .theta. is proportional to the vacuum pressure of the engine throttle which may represent the amount of depression of the acceleration pedal. The output torque T of the gasoline engine is proportional to the oil pressure of the hydrostatic transmission.
In order to maintain the engine operation along the line l, the drive ratio of the continuously variable drive ratio transmission may be so set that the difference between no and na is normally zero, no being the command rotational speed at which the line l and the line .theta. are crossed such as at the point P and na being the rotational speed at which the engine is actually operated at the corresponding throttle degree such as at the point a.
FIG. 2 shows how the dynamic brake is conventionally applied by releasing the acceleration pedal, as disclosed in U.S. Pat. No. 3,913,418 granted on Oct. 21, 1975 to Miyao and Sakai.
In FIG. 2, the vertical axis shows the output horse power (PS) of the gasoline engine, the horizontal axis shows the rotational speed n(rpm) thereof, the currves .theta. show throttle degree, and the line l shows the minimum fuel consumption as in FIG. 1. When the line l is within the zone above the horizontal axis, it means the engine is operated for driving, while when the line l is within the zone below the horizontal axis, it means the engine is operated for dynamic braking. As seen, the line l is continuously single from the drive zone to the dynamic brake zone so that the dynamic brake is applied when the throttle degree approaches .theta.o.
According to the method as disclosed in FIG. 2, however, when the line l is set so as to pass the point C wherein the maximum horse power is generated at the maximum throttle degree .theta.m and the point B wherein the maximum dynamic brake is applied at the minimum throttle degree .theta.o, the deflecting portion 2005 of the graph of the degree of throttle opening thereof becomes too sharp as seen. Therefore, it is practically difficult in the design of the engine to get the command engine rotational speed no at the corresponding throttle degree .theta.. In addition, the angle .alpha. between the throttle degree .theta. and the line l at the dynamic brake zone is relatively small so that a slight change of the throttle degree .theta. may unavoidably result in the considerable change of the command rotational speed no.